You must be cautious about inferring migration patterns from points on
a map without time tags. There is no doubt that a migration occurred
from Spain northward after the retreat of the European ice sheet, but
that was ages and ages before there was any such thing as "Celtic" or
even "proto-Celtic". It is certainly possible that the string of
matches you are seeing does reflect that ancient migration, but it
could also be much more recent (and could therefore be Celtic as you
suggest). It could also be pointing in either direction or even from
the middle outward. It could also be just an artifact of the database
due to the small sample size.